1. Technical Field
Embodiments relate to a back-side illuminated image sensor.
2. Discussion of the Related Art
FIG. 1 is a cross-section view schematically and partially showing an example of a back-side illuminated image sensor 1, formed inside and on top of a thinned semiconductor substrate 3. In this example, substrate 3 is of type P. Its thickness, for example, ranges between 1 and 10 μm.
Insulating regions 5, for example made of silicon oxide, extend from the front side (the upper surface in the orientation of the drawings) to the back side of substrate 3, perpendicularly to these surfaces. Regions 5 form partitions delimiting, in top view, a plurality of substrate portions 3a and 3b. Each substrate portion 3a comprises at least one photodiode and may comprise charge transfer devices (not shown), corresponding to a pixel of the sensor, and each substrate portion 3b comprises one or several control transistors (not shown). Regions 5 have the function of insulating substrate portions 3a and 3b from one another. They especially enable to avoid for electrons generated in a given substrate portion 3a, due to the illumination of this substrate portion, to be collected by a photodiode of another substrate portion 3a. 
The front surface of substrate 3 is coated with a stack 7 of insulating and conductive layers in which various interconnections of the sensor are formed. It should in particular be noted that stack 7 comprises contacting areas (not shown), enabling to bias substrate portions 3a and 3b to a common reference voltage.
The rear surface of substrate 3 is coated with a thin insulating layer 9, for example, made of silicon oxide, itself coated with an antireflection layer 11, for example formed of a stack of several transparent dielectric layers of different indexes. Antireflection layer 11 is covered with juxtaposed color filters, altogether forming a filtering layer 13. In the shown example, a first pixel is topped with a green filter (G) and a second neighboring pixel is topped with a blue filter (B). Microlenses 15 are stacked on filtering layer 13, to concentrate the received light rays towards the photodiodes.
A disadvantage of this type of sensor lies in the presence in the substrate of eddy currents, called dark currents, which may result in malfunctions and/or in altering the sensor performance. Dark currents are due to the spontaneous random generation of electron-hole pairs at the level of certain defects of the crystal structure of the substrate. Such current are capable of appearing even in the absence of any illumination of the sensor. They are more specifically generated at the interfaces between substrate 3 and insulating regions 5 and 9, which are areas with a great concentration of crystal defects.
To decrease dark currents, it has been provided to heavily dope, by implantation, strips of substrate 3 at the interfaces with regions 5 and insulating layer 9. However, the provision of these implantations has various disadvantages. In particular, the presence of a heavily-doped strip on the side of the substrate surface exposed to light alters the sensor sensitivity, especially in blue or ultraviolet wavelength ranges, where the photons are absorbed by very small substrate thicknesses. Further, the presence of such strips of heavy doping level, at the periphery of substrate portions 3a and 3b, tends to affect the behavior of the sensor transistors, and more specifically to inappropriately modify their threshold voltage. Further, the provision of an implantation on the rear surface side of the sensor raises technical issues.